A standard motor-vehicle door latch can be switched between a locked and unlocked position by operation of a standard lock cylinder by a mechanically and even magnetically bitted key. When the appropriately bitted key is inserted into the cylinder, it can be turned to move a locking lever of the latch between the locked and unlocked positions. In the locked position at least the outside door handle is either blocked or is uncoupled from a latching lever of the latch mechanism so that its actuation will not open the door. This system is standard and can be overcome in many ways, as for instance by picking or forcing the lock.
Accordingly more sophisticated systems have been proposed where the key has electrical coding, often constituted as a transponder right in the key that can be read by an appropriate device in the latch. The transponder reader will only issue a command to the latch allowing it to be moved between the locked and unlocked positions when a key having the appropriately coded transponder is inserted into the key passage. Such a system can be made very secure, as the number of codes is vast. It has, however, the major disadvantage that, if the vehicle's battery goes dead, the latch becomes nonoperational so some sort of mechanical backup that constitutes a security problem must be provided.
Accordingly German patent 4,023,386 of Hiebl describes a combined electrical/mechanical system where an electrical/mechanical key is used. When the electrical coding is correct, the latch establishes a mechanical connection so that the latch then functions as a mechanical key. The system monitors battery power and, if it detects a decline in voltage that indicates an impending power failure, it establishes the necessary mechanical coupling so that, even if the power fails completely, the latch can still be operated mechanically. Such a system offers certain advantages, but fails to function if power is cut suddenly. Furthermore the mechanical backup represents a weak aspect of the lock from the security point of view.